1. Field of the Invention
This invention relates generally to a color video signal processing system, and more particularly is directed to apparatus for correcting effects of angular errors in the chrominance subcarrier of a color video signal.
2. Description of the Prior Art
In apparatus for the magnetic recording and reproducing of color video signals, frequently referred to as a color VTR, an automatic phase control (APC) circuit or a phase-locked servo loop (PLL) is provided in order to eliminate variations in the frequency or phase of the subcarrier of the chrominance component of a color video signal reproduced from a magnetic tape. In such apparatus provided with an APC circuit or PLL, a burst signal is extracted from the reproduced chrominance component and is applied to a phase comparator where it is compared with a stable reference signal, for example, from a local oscillator, with the output or error voltage issuing from the comparator as a result of such comparison being employed for controlling the frequency and/or phase of the chrominance subcarrier.
When an APC circuit is employed, if the phase deviation or variation of the subcarrier frequency of the reproduced chrominance component is too great, the APC circuit cannot correct the subcarrier frequency to the desired value. However, if the phase variation of the subcarrier is within a predetermined range, the deviated subcarrier frequency can be captured or pulled in to the correct value. The frequency range within which the deviated subcarrier frequency can be pulled in to the correct value is referred to as the "pull-in range in APC. " After the subcarrier frequency has been once pulled in to the correct frequency, the frequency range within which it is possible to again correct the deviated frequency of the subcarrier is referred to as the "hold range in APC. " If the color video signal is in accordance with the NTSC system, and thus has a horizontal frequency of 15.734 KHz, the hold range may extend over .+-. 15.734 KHz, while the pull-in range may extend over one-half of the hold range that is, over .+-. 7.9 KHz. However, if the color video signal is in accordance with the PAL system, in which the phase of the burst signal is changed by 90.degree. in successive horizontal periods, it will be apparent that the frequency of the periods in which the burst signals are of the same phase is one-half the horizontal or line frequency. Thus, in the case of a color video signal according to the PAL system, the pull-in range and hold range are approximately one-half the corresponding ranges in the case of a color video signal according to the NTSC system. Therefore, since the horizontal frequency in the PAL system is 15.625 KHz, the hold range for a color video signal of such system may extend over .+-. 7.8 KHz while the pull-in range therefor may extend over .+-. 3.9 KHz.
As a result of the foregoing, if a color VTR is provided to selectively record and reproduce either NTSC or PAL color video signals, for example, as disclosed in detail in U.S. Pat. No. 3,854,237, issued Oct. 29, 1974, and having a common assignee herewith, and hold and pull-in ranges are made to be the same for reproducing both types of color video signals, the subcarrier frequency of the chrominance component may be mis-locked to an erroneous frequency when reproducing the PAL color video signals. In order to avoid the foregoing problem in a color VTR for recording and reproducing both PAL and NTSC color video signals, the hold and pull-in ranges, when reproducing PAL color video signals, have to be selected to be about one-half the values of the respective ranges when reproducing NTSC color video signals. To this end, it has been proposed in the prior art, when a PAL color video signal is being reproduced, the loop gain of the above mentioned APC circuit should be reduced about 6 dB, that is, to approximately one-half the loop gain of the APC circuit when reproducing an NTSC color video signal.
However, when the reduced loop gain of the APC circuit is employed during reproducing of a PAL color video signal, as proposed in the prior art, an undesirable phenomenon may occur. More specifically, due to the reduced loop gain of the APC circuit, when the phase comparator detects a deviation of the burst signal from the reference signal, the output or error voltage issuing from the comparator will exhibit a relatively slow rising rate with the result that the correction of the frequency and/or phase of the chrominance subcarrier may not be completed during the blanking interval and may carry over into the beginning of the subsequent vertical trace or scanning period. By reason of the foregoing, hue abbreviations may appear at the upper portion of the color television picture produced from the reproduced PAL color video signal. Moreover, the described problem arises in any color VTR which employs an APC with a relatively small or reduced loop gain for reproducing PAL color video signals, that is, in a color VTR intended only for use with PAL color video signals, as well as in a color. VTR intended for use with both NTSC and PAL color video signals.